Obesity is a major risk factor to develop the metabolic syndrome, characterized by hypertension, glucose intolerance, insulin resistance, dyslipidemia and increased propensity to develop diabetes type 2. A likely cofactor promoting the alarming increase in obesity in the last 10 years is the availability of food with high caloric and fat contnt. Exposure to a hypercaloric, high-fat (HF) diet induces Endoplasmic Reticulum (ER) stress and inflammation in regions of the hypothalamus controlling appetite. ?-MSH is the natural agonist of Melanocortin- 4 receptor (MC4R), a G-protein coupled receptor (GPCR) expressed by neurons of the hypothalamus that signals to decrease appetite. Because MC4R functions distally to control appetite, it has been considered as a most relevant target for anti-obesity therapies. However, even very potent MC4R agonists do not appear to treat obesity in mice and humans and the underlying mechanisms by which such agonists are ineffective are yet unclear. The overall hypothesis of this proposal is that lipid stress induces loss of MC4R function by altering the abundance (Aim 1 and Aim 2) and the traffic (Aim 3) of the receptor and that correcting such defects by chemical chaperones would facilitate weight loss by MC4R agonists. Aim 1 will determine whether adverse effects by increased palmitate on MC4R abundance observed in cultured neurons take place in the hypothalamus of mice exposed to HF-diet. The aim uses lentivirus-dependent delivery of an MC4R reporter to a region of the hypothalamus that controls appetite and measures abundance of endogenous MC4R in the hypothalamus by a mass spectrometry-based approach. Aim 2 will determine, by using biochemical and immunofluoresce-based assays, whether increased expression of transcription factors and chemical chaperones that modulate ER stress rescues MC4R abundance and function in cultured neurons exposed to elevated palmitate. The aim will also determine whether the combination of chemical chaperones and MC4R agonists promote reduced food intake and sustained weight loss in mice that are obese by being exposed to high fat diet. Aim 3 will determine whether exposure of immortalized hypothalamic neurons to elevated palmitate changes the cell lipid composition and traffic of MC4R to increase desensitization of the receptor upon prolonged exposure with the agonist, and whether such effects are blunted by a chemical chaperone. The aim will also determine whether administration of chemical chaperones restores hypothalamic lipid composition in mice obese because of HF diet (Aim 3). The proposed research will increase knowledge on an understudied topic, namely how lipid stress affects MC4R function and help identify new targets to treat obesity.